[S-IT15] Mass and energy transport properties and processes in the crust and the mantle

[SIT15-P06] Large scale recumbent folds and convex-upward accretionary prisms: excerpts from an experimental viscous model

*SREETAMA ROY^1,2、SANTANU BOSE³ (1.Earthquake Research Institute, The University of Tokyo、2.Department of Geology, University of Calcutta, India、3.Department of Geology, Presidency University, India)

キーワード：accretionary prism, Viscous model, recumbent folds , decollement strength

Accretionary prisms are wedge-shaped bodies of crustal rocks formed by compressive forces at convergent plate margins. Some of these accretionary prisms have an upward convex topographic curvature. Here we discuss one of the potential mechanisms of development of such an unusual topography using laboratory scale physical experiments. We used a viscous polymer (Polydimethylsiloxane or PDMS) to simulate the crust, which can easily accommodate pervasive and spatially variable deformation, especially in accretionary prisms where deformation is dominantly controlled by fluid or thermally activated creep. To generate compressive stress, a moving piston at a constant speed (~1.97 × 10^-4 ms^-1) was used and the convergence rate was kept uniform to annul the effects of rate dependence on viscous deformation. Strain analyses (PIVLAB and Ulysses 3D scans) from these crustal scale models coupled with some earlier theoretical findings imply that the ubiquitous convex-upward topography in accretionary wedges is characteristic of a strong basal decollement coupling beneath the wedge. The wedge attains a convex upward profile, which is similar in morphology to large crustal scale recumbent folds often reported from the interior of mountain belts. Therefore, we further investigated the morpho-metric evolution of recumbent folds in accretionary prisms using two-layered crustal scale models. In these models, we added an elastic sheet overlying the viscous slab (to simulate competence contrast between crustal layers) to show the development of folding in these models subject to compressional tectonic forces. Model results reveal conspicuous development of antiformal folds in the hinterland part of these wedges which eventually undergo rotation to develop a recumbent geometry. We further validated our results by physically varying the basal coupling strength (in strike parallel patches) below the crustal layer with steeper topographic profiles and recumbent folds developing over the strongly coupled decollement patches.